The present invention is directed to the field of electro-polishing metallic stents, with particular application to stents manufactured from nickel-titanium and stainless steel alloys.
Stents are used in conjunction with a medical procedure known as balloon angioplasty which is used to restore blood flow through obstructed or partially obstructed arteries. In an angioplasty procedure, a balloon catheter is inserted into an artery and advanced to the site of an arterial lesion. Subsequently, the catheter balloon is inflated to expand the artery and compress the accumulated atherosclerotic plaque which forms the arterial lesion against the artery wall, thereby restoring blood flow through the vessel. In a certain percentage of cases, the expanded artery will collapse after deflation of the catheter balloon or will slowly narrow over a period of time. To solve this problem, intravascular prostheses commonly referred to as stents have been developed.
A stent generally consists of a small, expandable, tubular metallic structure. There are two broad categories of stents in common use, self-expanding stents and balloon expandable stents. One type of self-expanding stent is a tubular member made from a nickel-titanium alloy. Alloys of this type possess what is commonly referred to as shape memory ability. Upon absorption of heat, these alloys will expand while undergoing a phase transformation from martensite to austenite. In nickel-titanium alloys used for stent applications, this phase change occurs at a temperature below the mean human body temperature of 98.6 degrees Fahrenheit. Typically, a nickel-titanium alloy stent is delivered to lesion site by means of a catheter designed to receive and chill the stent. Upon delivery to a lesion site, the stent expands radially to contact the artery walls upon being warmed to body temperature.
A balloon expandable stent by contrast is typically made from a deformable metallic material and is crimped onto the balloon portion of a balloon delivery catheter. Upon delivery to a lesion site, the stent is expanded radially, by inflating the catheter balloon, until it contacts the walls of an artery. With either type of stent, the expanded stent is left in place after withdrawal of the delivery catheter. The expanded stent supports the interior wall of the blood vessel and thereby prevents the blood vessel from collapsing or narrowing over time.
Stents are commonly manufactured from stainless steel, nickel-titanium alloys, and other materials. Stents are typically formed by machining selected patterns into drawn tubes of the desired material. The machining processes typically used are either Electro-Discharge Machining (xe2x80x9cEDMxe2x80x9d), which is based on the principle of erosion of metals by spark discharges, or Laser Beam Machining (xe2x80x9cLBMxe2x80x9d), which uses a narrow beam of light of high energy density to vaporize selected portions of the drawn metallic tube. Either machining process leaves a thin heat effected zone around the pattern cut in the drawn tube and a resulting surface finish that is coarse and unsuitable for implantation in living tissue. The surface finish of stents in the xe2x80x9cas machinedxe2x80x9d condition is on the order of about 50-100 microns, while stents suitable for implantation within a blood vessel require a surface finish of about 0.2 to 0.05 microns. A surface roughness of 0.2 microns corresponds to a fine buff finish, while a surface roughness of 0.05 microns corresponds to a mirror-like polish.
To achieve the required surface finish, stents are typically descaled and electro-polished. After polishing, the stents are typically passivated to protect the polished surface. One method of descaling involves immersing the stents in an alkaline cleaner and ultrasonically agitating the stents for a selected period of time. Another method involves bead blasting stents with fine glass beads. There are other procedures for descaling which are well known to those skilled in the art.
The principles of electro-polishing, particularly with regard to stainless steel alloys, are also known in the art. Typically, an item to be electro-polished is immersed in an electrolyte which comprises an aqueous acidic solution. The item to be polished is made a positive electrode (anode) and a negative electrode (cathode) is placed in close proximity to the anode. The anode and cathode are connected to a source of electric potential difference with the electrolyte completing the circuit between anode and cathode. Upon the passage of electric current through the electrolyte, metal is dissolved from the anode surface with protrusions being dissolved faster than depressions, thereby producing a smooth surface. The rate of material removal in an electro-polishing process is primarily a function of the electrolyte chosen and the current density in the electrolyte fluid.
Typically, with stainless steel stents, a final step in the electro-polishing process involves passivation of the newly polished surface. After removal from the electrolyte solution and rinsing with water, residual anions of the acid used in the electrolyte remain in contact with the polished surface. The presence of such surface anions leads to deterioration of the newly polished surface when the residual anions come into contact with calcium and magnesium ions which are commonly found in non-deionized water (ordinary tap water). To prevent surface deterioration, newly polished stents are immersed in a passivation bath which typically consists of a solution of nitric acid, deionized water, and sodium dichromate. The passivation bath neutralizes the residual anions and leaves a protective, corrosion resistant, strongly adherent, transparent, chromium dioxide coating on the newly polished surface.
With nickel-titanium alloy stents, however, the passivation step is generally not required. Nickel-titanium alloys tend to form a titanium oxide rich surface layer during initial heat treatment of the alloy which renders the alloy relatively impervious to the corrosive effects of any residual anions that may be left on the stent surface after electro-polishing.
Although electro-polishing has proven to be an effective method of obtaining extremely smooth surfaces on the order of 0.2 to 0.05 microns surface roughness on relatively flat surfaces, simple immersion in an electrolytic bath has proven ineffective in obtaining a uniform degree of polish on both the interior and exterior surfaces of a tubular stent. What is needed to solve this problem is a stent polishing fixture that is able to maintain a uniform current density in the electrolyte solution within the interior of a stent as well as a uniform current density in the electrolyte solution about the exterior of a stent. In addition, electrolyte solutions specifically formulated for use with nickel-titanium alloys are needed as typical solutions formulated for use with stainless steel, generally have not been able to achieve the desired surface finish. The present invention satisfies these and other needs.
The present invention provides an electro-polishing fixture for providing a fine uniform polish on both the interior and exterior surfaces of metallic stents. The fixture achieves this result by providing a plurality of anodes around the circumference of a stent thereby creating uniform current density in the surrounding electrolyte when the stent is place within an electrolytic bath. The fixture of the present invention further includes a center cathode centrally located within the interior of a stent, which provides for uniform polishing of a stent""s interior surface. The invention also includes a curved exterior cathode disposed about the perimeter of the stent which provides for uniform polishing of a stent""s exterior surface. The combination of dual interior and exterior cathodes in conjunction with a plurality of anodes provides a high degree of uniformity in polishing.
The present invention further includes an electrolyte solution which is particularly well suited for producing a fine polish on stents made from nickel-titanium alloys and a method of using the electro-polishing fixture and the electrolyte of the present invention to polish nickel-titanium alloy stents. Other features and advantages of the present invention will become apparent from the following detailed description.